In some cases, they make host plants produce a compound that’s toxic to insects. Researchers also have created crops able to withstand herbicides that typically would kill them. This tweak allows growers to douse weeds without harming their harvest.

Pest- and herbicide-tolerant varieties have been wildly popular among farmers since their introduction in 1996. They’re now grown on more than 300 million acres annually worldwide.

Some biotech backers said the public soon will see plants that need less water and fertilizer, allowing them to flourish in arid regions and places where the soil is too poor for conventional crops.

Locally, about 15 scientists have formed the San Diego Center for Molecular Agriculture to help advance the movement. Their work centers on finding the genes that trigger certain plant responses and not on creating commercially viable products, said Maarten Chrispeels, an emeritus professor of biology at the University of California San Diego.

“These days, there is much to do about getting plants to extract nutrients from the soil more efficiently,” he said.

It’s an alluring goal for farmers, humanitarian groups and companies that patent various plant traits.

For years, they have touted the potential of genetically engineered rice that’s rich in beta-carotene — a precursor to vitamin A — as a way to decrease malnutrition in Asia. This “golden rice,” named for its distinctive yellow color, has drawn international attention for more than a decade but still hasn’t been released to the public.

Scientists have found it time-consuming to get the vitamin A trait into local rice varieties, and it takes years to gain government clearance for biotech crops. Regulations are abundant because many people worldwide are uneasy about the potential for lab-formed plants to create health and environmental problems that don’t show up until they have spread widely.

Chrispeels said fears about genetically engineered food have calmed considerably in the United States during the past few years, despite continued angst in Japan and Europe.

“We have had biotech crops for 15 years now and they are not going away, even if the Europeans don’t like them,” he said.

His enthusiasm aside, Chris-peels isn’t convinced that companies will produce many crops offering benefits to consumers.

“The emphasis will remain the way it was — things that benefit production,” he said. “There are so many ways of getting a healthy diet, you don’t actually need transgenics.”

Other scientists are much more critical of the technology, saying it has failed to deliver on claims that it could feed the world and reduce the ecological damage done by farming.

“People who have been listening to these promises for a long time would have reason to be skeptical,” said Margaret Mellon, director of the food and environment program at the Union of Concerned Scientists in Washington, D.C.

A study published in April by Mellon’s group concluded that worldwide food production exceeds demand, calling into question the need for more biotech crops. It said gains in agricultural productivity are largely due to improvements in conventional breeding and other farming techniques — not biotechnology.